The present invention relates to a piezoelectric material extensively used in the fields of actuator, sensor, resonator, etc.
A piezoelectric material is a material having an effect of generating strain when an electric field is applied from outside (conversion of electrical energy to mechanical energy) and an effect of generating an electrical charge on the surface thereof when a stress is applied thereto from outside, and extensively in use in the fields of actuator, sensor and the like. For instance, a piezoelectric material of PZT type (lead zirconate titanate) generates a strain roughly proportional to the input voltage at order of 10.sup.-10 m/V, and therefore is successfully applicable to microscopic adjustment of position, etc. and utilized in the fine adjustment of optical systems. Also, since it generates electrical charge of which magnitude is proportional to the applied stress or deformation of piezoelectric material itself, it is utilizable as a sensor for reading out a minute force or deformation. Further, since a piezoelectric material is excellent in responsibility, it is also possible to excite a piezoelectric material itself or an elastic material in a conjunctive relation with a piezoelectric material and thereby to cause a resonance by applying an alternating electric field.
Most of the piezoelectric materials practically in use today belong to a solid solution system (a PZT system) consisting of PbZrO.sub.3 (PZ)--PbTiO.sub.3 (PT). This is for the reason that a material of excellent piezoelectric characteristics can be obtained by using a composition close to the crystallographic morphotropic phase boundary (M.P.B.) between trigonal PZ system and tetragonal PT system. In the field of PZT, products of wide composition range capable of replying the various needs have been developed by adding various additives or subsidiary components to M.P.B. composition. The products thus developed cover a wide variety of ones ranging from those used in position-adjusting actuator capable of indicating a large displacement when used in a direct current because of small mechanical quality factor (Qm) and great piezoelectric constant (d33) to those suitable for use in alternating current such as ultrasonic wave-emitting element or ultrasonic motor or the like because of small piezoelectric constant (d33) and large mechanical quality factor (Qm).
In addition to the above, most of the piezoelectric and electrically strainable materials practically used currently have a solid solution composition of which main component is lead type perovskite composition such as PZT, PMN (lead magnesium-niobate) or others.
Now, these lead type piezoelectric materials contain so large an amount of lead oxide (PbO) as about 60-70% by weight, showing a high volatility even at low temperatures, as main component. For instance, in PZT and PMN, about 2/3 by weight of the total weight is occupied by lead oxide. In the manufacture of ceramics or single crystal of these piezoelectric materials, heat treatments such as firing, fusion, etc. are indispensable processes. When these heat-treatments are carried out on an industrial scale, a large quantity of the volatile lead oxide is lost by vaporization or diffusion into atmospheric air. Although the lead oxide released in the process of manufacture can be recovered, the lead oxide present in the industrial piezoelectric materials released into the market can hardly be recovered at the present time. If such a large quantity of lead oxide is released into environments, it unavoidably causes pollution of environment in the form of, for instance, acidic rain, etc. In view of the enlargement of application field of piezoelectric ceramics and single crystals and increased consumption thereof, the problem of development of pollution-free or lead-free piezoelectric material is very important.
As piezoelectric materials entirely free from lead, there are known BaTiO.sub.3 and Bi-layer ferroelectric substances. However, the former has so low a Curie temperature as 120.degree. C. above which its piezoelectricity disappears, so that this type of piezoelectric material is not practical for uses in cars or/and the like. The latter usually has a Curie temperature of above 400.degree. C., and is superior in heat stability. However, it is high in crystalline anisotropy and its spontaneous polarization has to be oriented by the method of hot forging or the like, which is undesirable from the viewpoint of productivity. Further, if the content of lead is made to zero, a high piezoelectricity is difficult to obtain.